Applications of optical memory technology using optical disks as high density and high capacity information recording media are expanding to digital audio disks, video disks and document file disks and also to data files, and entering mainstream use. In order successfully to achieve highly reliable recording and reproduction of information onto optical disks via a highly stopped down light beam, a focusing function that forms a minute spot at the diffraction limit, and optical system focusing control, tracking and pit signal (information signal) detection functions are necessary.
In recent years, the development of optical disks with high density recording capacity that is greater than that of conventional optical disks has advanced due to the advancement of optical system design technology and a reduction in the wavelengths of semiconductor lasers serving as the light source. Increasing the size of the optical disk side numerical aperture (NA) of the focusing optical system that stops down the light beam to a minute point also has been investigated as another approach to increase the density.
Compact disks (CDs), which may be considered first generation optical disks, use infrared light (with a wavelength λ3 of 780 nm-820 nm) and an objective lens having an NA of 0.45, and the substrate thickness of the disks is 1.2 mm. Second generation DVDs use red light (with a wavelength λ2 of 630 nm-680 nm) and an objective lens having an NA of 0.6, and the substrate thickness of the disks is 0.6 mm. For third generation high density disks, a system using blue light (with a wavelength λ1 of 380 nm-420 nm) and an objective lens having an NA of 0.85, and having disk substrate thicknesses of 0.1 mm is proposed.
It should be noted that in the present application, substrate thickness refers to the thickness of the transparent substrate from the surface at which the light beam is incident on the optical disk (or information recording medium) to the information recording surface.
In this manner, the substrate thickness of the optical disk gets thinner as the recording density increases. Furthermore, dual layering of the recording layer is carried out as another method for realizing higher densities. For DVDs, dual-layer disks are the standard for read only ROM disks, and for high density third generation disks, dual-layer disks have also been proposed for recordable disks.
An optical disk appatus that can record and reproduce optical disks having different substrate thicknesses and recording densities is desirable from the point of view of economics, and the space that the apparatus occupies. Thus, it is necessary to have an optical head device that is provided with an optical detection system that can detect light of different wavelengths that are irradiated onto optical disks.
Detection is possible if individual photodetectors are provided for each different wavelength of light. However the optical system thus becomes complex, the flexible cable for outputting signals from the detectors also becomes complicated, and there is an increase in cost.
In the case of recording and reproducing different types of optical disks, a conventional example in which a single photodetector is shared is proposed in, for example, Patent Reference 1 below. However, in this example it is a prerequisite that focus detection is by an astigmatic aberration method, and when using disks such as DVD-RAM in which the groove pitch is relatively large compared to the light spot on the disk, there have been problems such as external interference affecting the focus signal when transversing the tracks, causing the focus servos to become unstable.
Furthermore, the optical heads up to now have not taken into consideration multi-layer disks having a plurality of formats, and countermeasures for offset fluctuations due to light scattering from other layers have not been implemented.
The details proposed in Patent Document 1 hereby are described briefly with reference to FIGS. 18 and 19. FIG. 18 shows a structural overview of an optical head 1. FIG. 18A shows a state of the optical head 1 when recording and reproducing information on a DVD, and FIG. 18B shows how the optical head 1 records and reproduces information onto a CD. The optical head 1 contains a red semiconductor laser 2 that generates light having a wavelength of 650 nm, and an infrared semiconductor laser 3 that generates light having a wavelength of 780 nm.
First, the case in which a DVD disk is reproduced is described. Light generated by the red semiconductor laser 2 passes through a wavelength selection prism 4 and is converted to parallel light by a collimating lens 5. The light that is converted to parallel light is reflected by a beam splitter 6, passes through a wavelength filter 7 and a ¼ wavelength plate 8, is converted to convergent light by an objective lens 9, and is irradiated onto a DVD disk 10. The light that is reflected/diffracted by the DVD disk 10 again passes through the objective lens 9, the ¼ wavelength plate 8 and the wavelength filter 7, passes through the beam splitter 6, is diffracted by a hologram 11 so as to be converted to convergent light and is focused on a photodetector 12.
Next, the case in which a CD disk is reproduced is described. Light that is generated from the infrared semiconductor laser 3 is reflected by the wavelength selection beam splitter 4, and is converted to parallel light by the collimating lens 5. The light that has been converted to parallel light is reflected by the beam splitter 6, passes through the wavelength filter 7 and the ¼ wavelength plate 8, is converted to converging light by the objective lens 9 and is irradiated onto a CD disk 13. The light that is reflected by the CD disk 13 passes again through the objective lens 9, the ¼ wavelength plate 8 and the wavelength filter 7, passes through the beam splitter 6, is diffracted by the hologram 11 to be converted to convergent light, and is focused on the photodetector 12.
As shown in FIG. 19A, the hologram 11 is divided into a plurality of regions, and one part of the regions (H1) guides only red light to the photodetector 12 and the other part of the regions (H2) guides only infrared light to the photodetector 12. In doing so, both regions cause the light to converge, as well as impart astigmatic aberration. As shown in FIG. 19B, the photodetector 12 has four detection regions. The light having astigmatic aberration is irradiated onto the center of the four detection regions.
In this mode, a focus error signal for the astigmatic aberration method is created by the difference between a sum of diagonally opposite regions (A+C) and a sum of the other diagonally opposite regions (B+D), of the four detection regions. Furthermore, for a tracking signal, a tracking error signal according to the push pull method is created from the difference between a sum of the regions on the same side (A+B) with respect to the track projection and a sum of the regions on the other side (C+D).
Furthermore, a tracking error signal for the phase differential method is created by comparing the phases of the sum of the diagonally opposite regions (A+C) with phases of the sum of the other diagonally opposite region (B+D). Moreover, an RF signal, which is a reproduction signal, is created from the sum of the entire region.
It is required that this optical head device reliably detect information recording media, whose applicable wavelengths differ and which include multi-layer disks, using a minimum of photodetectors.
However, in this configuration, when reproducing multi-layer disks such as dual-layer disks, scattered light from layers that are not the layer that is to be read is incident on the photodetector, accordingly these offset the original signal, and thus there has been the problem that a reliable signal could not be detected. In this case, it has been necessary to increase the number of photodetectors in order to avoid scattered light.
Patent Reference 1
JP 2002-216385A (First diagram)